1. Field of the Invention
The present invention relates to a method for manufacturing a thin film transistor and a liquid crystal display device.
2. Description of the Related Art
In recent years, reduction in cost, thickness, and weight of liquid crystal display devices has been greatly needed.
As one of methods for achieving cost reduction of liquid crystal display devices, simplification of a manufacturing process of liquid crystal display devices can be given.
Driving methods of liquid crystal display devices are broadly classified into a passive matrix method and an active matrix method. In recent years, active matrix liquid crystal display devices which are excellent in image quality and high-speed response have been in the mainstream.
In an active matrix liquid crystal display device, each pixel is provided with a switching element. As the switching element, a thin film transistor is mainly used. As such a thin film transistor, a top-gate transistor whose channel formation region is provided below a gate electrode and a bottom-gate transistor whose channel formation region is provided over a gate electrode are given. These thin film transistors are generally manufactured using at least five photomasks.
Reducing the number of photomasks as much as possible is one of important factors to manufacture liquid crystal display devices at lower cost. In order to reduce the number of photomasks, a complicated technique such as backside light exposure (for example, see Patent Document 1), resist reflow, or a lift-off method, which requires a special apparatus, is used in many cases. The use of such a complicated technique might lead to various problems such as reduction in yield of liquid crystal display devices and degradation of electric characteristics of thin film transistors.
Further, as one of methods for achieving reduction in thickness and weight of liquid crystal display devices, reducing the thicknesses of substrates between which a liquid crystal material is sandwiched, by mechanical polishing, chemical polishing, or the like, are given.
Mainly, glass substrates are used as substrates between which a liquid crystal material is sandwiched and thus, there is a limit on reduction in thicknesses of such substrates by mechanical polishing, chemical polishing, or the like. Moreover, there is a problem that as the thicknesses of such substrates are reduced, the strength of the substrates is lowered and a liquid crystal display device is more likely to be damaged by external impact. Therefore, it is ideal to manufacture a liquid crystal display device with the use of significantly tough supports (such as a resin film and a metal film) as substrates between which a liquid crystal material is sandwiched.